This invention relates to a brake pressure reducing valve assembly for use in a vehicle braking system to supply, under certain braking conditions, a brake pressure to the rear brake actuators of the vehicle which is lower than the brake pressure supplied to the front brake actuators.
It is well recognized that it is a desirable characteristic for a vehicle braking system for the rear brake pressure to rise at the same rate as the front brake pressure as the brakes are applied until a certain threshold pressure (referred to as the cut-in pressure) is reached, and thereafter for the rear brake pressure to rise more slowly than the front brake pressure as the brakes are further applied. This desirable result can be achieved by interposing one or more pressure reducing valves into the brake supply line to the rear brakes.
Various embodiments of brake pressure reducing valve assembly are described in our co-pending International patent application No. PCT/GB90/00707, publication No. WO 90/13462 entitled "Brake Pressure Reducing Valve Assembly" incorporated herein by reference. The brake pressure reducing valve assemblies described in this application are particularly suitable for braking systems in which a single fluid pressure supply line is used to supply brake pressure to all the actuators of the rear brakes of a vehicle. Under these circumstances, any of the illustrated brake pressure reducing valves may be inserted into the rear brake pressure line to produce the desired control of rear brake pressure.
In the case of vehicles having two separate pressure lines which supply fluid to rear actuators, for example vehicles having so-called "X-Split" braking systems, two brake pressure reducing valves of the type described in WO 90/13462 would be required--one such valve assembly being inserted in each pressure line to the rear brakes. Such an arrangement would not be ideal because variations in manufacturing tolerance and the degree of wear between the brake pressure reducing valves could result in different brake pressures being supplied to the respective rear braking circuits. Further, each brake pressure reducing valve absorbs a certain volume of pressurized working fluid if the vehicle is laden and the brake system pressure rises above the driver only cut-in pressure. If two separate valves are used, this could result in an excessive requirement for pressurized working fluid.